Non-Dispersive Infra-Red (NDIR) is a common and excellent measurement technique for detecting gases in the atmosphere. NDIR sensors utilize the principle that various gas molecules exhibit substantial absorption at specific wavelengths in the infrared radiation spectrum. The term “non-dispersive” as used herein refers to the apparatus used, typically a narrow-band optical or infrared transmission filter, instead of a dispersive element such as a prism or diffraction grating. The optical filter isolates the radiation in a particular wavelength band that coincides with a strong absorption band of a gas species for the purpose of said gas species measurement.
For detecting molecules in liquids, the NDIR measurement method, which works well in the gaseous phase where the molecular density is low, encounters debilitating measurement noise caused by scattering because of the much higher molecular density in liquids. As disclosed by Wong and Campbell in U.S. Pat. No. 9,606,053 (2017), a method is advanced which significantly suppresses the noise attributable to scattering in the liquids. For determining the concentration of targeted molecules labeled M in a liquid sample admixed with interfering molecules labeled MJ which overlap the absorption band of molecules M, a unique sampling methodology for reducing the interference noise uses an additional interference radiation source besides those of the signal and reference is further advanced by Wong and Campbell in U.S. Pat. No. 9,606,053 (2017). The present invention is to extend the use of this unique sampling methodology to quantify the extent of the inaccuracy caused by the interfering molecules and to rectify the calibration curve for ensuring its future applicable validity.